This is one of the classic texts of the Négritude movement, which valorized black culture and identity. In this part of the long poem, Césaire, who is Martinican, is remembering the Haitian revolutionary Toussaint Louverture, considered a hero of black liberation.

What is mine
a lone man imprisoned in whiteness
a lone man defying the white screams of white death
(TOUSSAINT, TOUSSAINT LOUVERTURE)
a man who mesmerizes the white sparrow hawk of white death
a man alone in the sterile sea of white sand
a coon grown old standing up to the waters of the sky
Death traces a shining circle above this man
death stars softly above his head
death breathes, crazed, in the ripened cane field of his arms
death gallops in the prison like a white horse
death gleams in the dark like the eyes of a cat
death hiccups like water under the Keys
death is a struck bird
death wanes
death flickers
death is a very shy patyura
death expires in a white pool of silence (16-17)

Toussaint Louverture was a pretty awesome guy. Born a slave but later freed, he led the Haitian Revolution of 1791-1804, which forced the French out of Haiti. Césaire is giving a shout-out to Louverture here. After all, he led one of the first—and earliest—successful revolts against colonial rule and slavery.

This is poetry, so imagery and repetition are very important. "White" and "whiteness" are repeated a lot. Toussaint Louverture is "imprisoned" by whiteness. In other words, he's imprisoned by colonialism and slavery.

"Death" is another word that's repeated a lot. These lines show Louverture defeating death (and whiteness, which seems to be almost the same thing). No small achievement, for sure.